84
第
十
章
Ap
p
en
d
ix
I
Appendix I RS485 Communication protocol
I-1
Communication protocol
I-1-1
Communication content
This serial communication protocol defines the transmission information and use format in the
series communication Including: master polling( or broadcast) format; master encoding method,
and contents including: function code of action, transferring data and error checking. The response
of slave also adopts the same structure, and contents including: action confirmation, returning the
data and error checking etc. If slave takes place the error while it is receiving information or cannot
finish the action demanded by master, it will send one fault signal to master as a response.
Application Method
The inverter will be connected into a “Single-master Multi-slave” PC/PLC control network
with RS485 bus.
Bus structure
(1)Transmission mode
Asynchronous series and half-duplex transmission mode. For master and slave, only one of
them can send the data and the other only receives the data at the same time. In the series
asynchronous communication, the data is sent out frame by frame in the form of message
(2)Topological structure
Single-master and multi-slave system. The setting range of slave address is 0 to 247, and 0
refers to broadcast communication address. The address of slave for network must be exclusive.
Figure I-3 is the single inverter and PC set up MODBUS field wiring diagram. Because
computers are generally not with RS485 interface, the computer must be built-in RS232 interface
or USB interface through the converter to convert to RS485. Connect the T + of converter with 485
+ terminal of the inverter, Connect the T- of converter with 485- terminal of inverter. We
recommended to use a shielded twisted pair. When adopting the RS232-485 converter,RS232
interface connected with RS232-RS485 RS232 interface, the cable should be as short as
possible,15meters at the longest, we recommend to plug the RS232-RS485 with computer in pair
directly. Similarly, when using the USB-RS485 converter, cable should be as short as possible.
When the line is connected, connect the right port of the host computer on the computer to
(RS232-RS485 converter port, such as COM1), and set the basic parameters and the baud rate and
data bit parity and so on consistent with the inverter.
RS232 to RS485 converter
T+
T-
485 485+
RS232 cable 15m
at the longest
Shielded twisted pair
Frequency inverter
PE
-
Figure I-3
Multiple Applications
In reality, multi-machine applications, there are two connections
The first inverter and the last inverter short the terminal resistor on the control board to be active.
As shown in Figure I-4
